NOx and particulate matter (soot) emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit these emissions. Manufacturers and researchers have put considerable effort toward meeting those regulations. Diesel particulate filters (DPFs) have been proposed for controlling the particulate matter emissions. A number of different solutions have been proposed for controlling the NOx emissions.
In gasoline-powered vehicles that use stoichiometric fuel-air mixtures, NOx emissions can be controlled using three-way catalysts. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
One set of approaches for controlling NOx emissions from diesel-powered vehicles involves limiting the creation of pollutants. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful in reducing NOx emissions, but these techniques alone are not sufficient. Another set of approaches involves removing NOx from the vehicle exhaust. These approaches include the use of lean-burn NOX catalysts, selective catalytic reduction (SCR), and lean NOX traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. A reductant such as diesel fuel must be provided, which introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
To clarify the state of a sometimes ambiguous nomenclature, one should note that in the exhaust aftertreatment art the terms “SCR catalyst” and “lean NOx catalyst” can be used interchangeably. Often, however, the term “SCR” is used to refer just to ammonia-SCR, in spite of the fact that strictly speaking ammonia-SCR is only one type of SCR/lean NOx catalysis. Commonly, when both ammonia-SCR catalysts and lean NOx catalysts are discussed in one reference, SCR is used in reference to ammonia-SCR and lean NOx catalysis is used in reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons.
LNTs are devices that adsorb NOx under lean exhaust conditions and reduce and release the adsorbed NOx under rich exhaust conditions. An LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals, such as Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. In a reducing environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time to time to remove accumulated NOx and thereby regenerate (denitrate) the LNT.
During denitration, much of the adsorbed NOx is reduced to N2, although a portion of the adsorbed NOx is released without having been reduced and another portion of the adsorbed NOx is released after being deeply reduced to ammonia. U.S. Pat. No. 6,732,507 describes a system in which an SCR catalyst is configured downstream from the LNT in order to utilize the ammonia released during denitration. The ammonia is utilized to reduce NOx slipping past the LNT and thereby improves conversion efficiency over a stand-alone LNT.
In addition to accumulating NOx, LNTs accumulate SOx. SOx is the combustion product of sulfur present in ordinarily fuel. Even with reduced sulfur fuels, the amount of SOx produced by combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere. The temperature of the exhaust can be elevated by engine measures, particularly in the case of a lean-burn gasoline engine. At least in the case of a diesel engine, however, it is often necessary to provide additional heat. Once the LNT is sufficiently heated, then a reducing environment similar to LNT denitration is created.
Except when the engine can be run stoichiometric, or rich, creating a reducing environment for LNT regeneration generally involves injecting reductant into the exhaust. A portion of the reductant is required to eliminate excess oxygen from the exhaust. The amount of oxygen to be removed by reaction with reductant can be reduced in various ways, for example, by throttling the engine air intake. At least in the case of a diesel engine, however, it is generally necessary to eliminate a substantial amount of oxygen from the exhaust by combustion or reforming reactions with injected reductant. Reductant is also commonly injected into the exhaust to heat the LNT for desulfation or to heat a DPF to initiate soot combustion.
Reductant can be injected into the exhaust by the engine fuel injectors. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. A disadvantage of this approach is that engine oil can be diluted by fuel passing around piston rings and entering the oil gallery. Additional disadvantages of cylinder reductant injection include having to alter the operation of the engine to support LNT regeneration, excessive dispersion of pulses of reductant, and forming deposits on turbocharger and EGR valves. As an alternative to using the engine fuel injectors, reductant can be injected into the exhaust downstream from the engine using separate exhaust line fuel injectors. Injecting the exhaust directly into the exhaust line has the advantage of allowing the point of introduction to be selected.
An oxidation catalyst or a fuel reformer may be used within the exhaust line to combust or reform the injected reductant upstream from a pollution control device. U.S. Pat. No. 7,082,753 (hereinafter “the '753 patent”) describes an exhaust aftertreatment system with a fuel reformer placed in the exhaust line upstream from an LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen from the exhaust and converts the diesel fuel reductant into more reactive reformate. The inline reformer of the '753 patent is designed to heat rapidly and to then catalyze steam reforming.
Temperatures from about 500 to about 700° C. are required for steam reforming. These temperatures are substantially higher than typical diesel exhaust temperatures. To achieve a sufficient reformer temperature when LNT regeneration is required, the reformer of the '753 patent is heated by first injecting fuel at a rate that leaves the exhaust lean, whereby the injected fuel combusts in the reformer, releasing heat. After warm up, the fuel injection rate is increased to provide a rich exhaust. Ideally, the reformer of the '753 patent can be operated auto-thermally, with endothermic steam reforming reactions balancing exothermic combustion reaction. In practice, however, at high exhaust oxygen concentrations the reformer unavoidably and excessively heats if reformate is produced continuously. To avoid overheating, the '753 patent proposes pulsing the fuel injection.
U.S. Pat. No. 6,006,515 suggests that an LNT may be regenerated more efficiently by either longer or shorter chain hydrocarbons, depending on the LNT composition and the temperature at which regeneration takes place. In order to be able to control the selection between long and short chain hydrocarbons, the patent proposes two fuel injectors, one in the exhaust manifold upstream from the turbocharger and one in the exhaust line immediately before the LNT. Due to the high temperatures in the exhaust upstream from the turbocharger, fuel injected with the manifold fuel injector is said to undergo substantial cracking to form shorter chain hydrocarbons.
Diesel particulate filters must also be regenerated. Regeneration of a DPF is to remove accumulated soot. Two general approaches are continuous and intermittent regeneration. In continuous regeneration, a catalyst is provided upstream of the DPF to convert NO to NO2. NO2 can oxidize soot at typical diesel exhaust temperatures and thereby effectuate continuous regeneration. A disadvantage of this approach is that it requires a large amount of expensive catalyst.
Intermittent regeneration involves heating the DPF to a temperature at which soot combustion is self-sustaining in a lean environment. Typically this is a temperature from about 400 to about 700° C., depending in part on what type of catalyst coating has been applied to the DPF to lower the soot ignition temperature. A typical way to achieve soot combustion temperatures is to inject fuel into the exhaust upstream from the DPF, whereby the fuel combusts generating heat in the DPF or an upstream device.
Various exhaust line reductant injection systems for injecting diesel fuel for LNT regeneration have been proposed. A common issue addressed by these systems concerns the heat of the exhaust line. Heat from the exhaust line can cause fuel that remains stagnant within the fuel injectors between fuel injections to decompose into substances that eventually clog the fuel injectors.
One approach is to physically separate the injector from the exhaust line by providing a relatively long line between the injector and a point of entry into the exhaust line. A difficulty with this approach is that the injector is generally designed to provide the fuel in finely distributed droplets. These droplets may recombine within the relatively long line before reaching the point of entry.
Another approach is to design exhaust line fuel injectors with cooling jackets. Water or air cooling can be used. Alternatively, an injector can be cooled with the reductant being injected; an excess flow of reductant is provided to the injector. The excess flow is returned to a reservoir. The return flow carries away heat.
Robert Bosch GmbH has proposed an exhaust line fuel injection system with a separate metering valve and injection unit. The injection unit is a simple nozzle surrounded by a cooling jacket. The metering valve, which comprises a pulse width modulated (PWM) pulse width modulated valve, is kept some distance away from the exhaust line to protect temperature sensitive components of the valve, such as electrical insulators. The metering valve is designed to draw fuel from the low pressure portion of the engine fuel injection circuit or from a separate pump and/or pressure regulator. The flow rate is regulated through the duty cycle of the metering valve.
The Bosch system is configured with two pressure measuring devices, one upstream from the PWM valve and the other downstream from the PWM valve. A conventional way to control the flow through this system would be to relate the pressure drop across the metering valve together with the duty cycle of the valve to the flow rate. The duty cycle (the fraction of time the valve is open) can be increased or decreased until the desired flow rate is reached.
In spite of advances, a long felt need continues for an exhaust aftertreatment system that is durable, is reliable, has acceptable manufacturing and operating costs, and can reduce NOx emissions from diesel engines enough to meet U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.